Graphene is a flat two-dimensional sheet of carbon atoms arranged on d hexagonal lattice resembling a honeycomb, with two atoms per unit-cell [K. S. Novoselov, et al. Science 306,666 (2004), A. K., Geim, K. S. Novoselov, Nat. Mat. 6 (2007) 183, Y. B. Zhang, Y. W. Tan, H. L. Stormer, and P. Kim, Nature 438, 201 (2005)]. Carbon atoms are in the sp2-hybridized state. Each of them is attached to three other carbon atoms by sigma type bonding. The electronic structure of graphene is rather different from usual three-dimensional materials. Its Fermi surface is characterized by six double cones. In intrinsic (undoped) graphene the Fermi level is situated at the connection points of these cones. Since the density of states of the material is zero at that point, the electrical conductivity of intrinsic graphene is quite low. The Fermi level can however be changed by an electric field so that the material becomes either n-doped (with electrons) or p-doped (with holes) depending on the polarity of the applied field.
Close to the Fermi level the dispersion relation for electrons and holes is linear. Since the effective masses are given by the curvature of the energy bands, this corresponds to zero effective mass. The equation describing the excitations in graphene is formally identical to the Dirac equation for massless fermions which travel at a constant speed. The connection points of the cones are therefore called Dirac points.
Numerous experiments conducted in recent years confirmed that electrons in graphene behave like Dirac fermions, being characterized by an anomalous quantum effect, and that transport in graphene is of a ballistic nature [M. L. Sadowski, G. Martinez, M. Potemski, C. Berger, and W. A. de Heer, Phys. Rev. Lett. 97, 266405 (2006, D. L. Miller, K. D. Kubista, G. M. Rutter, et al., Science 324, 924 (2009)]. The exceptional electron properties of graphene and its high chemical stability make it a particularly attractive candidate for future electronics devices [Novoselov K. S., Geim A. K., Nature Materials 6, 183 (2007)]. Carriers mobility in graphene is significantly high, reaching up to 200000 cm2/Vs, which is more than one order of magnitude higher than in the case of silicon transistors [Lin Y. M. et al, Science 327, 662 (2010)]. This ensures ballistic transport over distances of the order of several micrometers. In addition, current density in graphene stays over 100 times higher than in copper (108 A/cm2) [M. Wilson, Phys. Today, p. 21 (January 2006)].
Graphene can be obtained by several methods. The first one, developed by K. S. Novoselov and A. K. Geim, is the mechanical exfoliation of bulk graphite with a strip of scotch tape until single layer of graphene is obtained. The produced flakes exhibited outstanding high carriers mobility. Since this method allows making only small size samples of graphene, (from few hundred to several thousand square micrometers) and an inefficient flakes selection process. It was not a practical method for a mass production.
The second method, devised by W. de Heer i C. Berger [C. Berger, Z. Song, T. Li, et al., J. Phys. Chem., B 108, 19912 (2004), W. A. de Heer. C. Berger, X. Wu, et al, Solid State Commun, 143, 92 (2007), K. V. Emtsev et al., Nat. Mater, 8, 203 (2009)] on the basis of earlier reports [A. J. Van Bommel, J. E. Crombeen, and A. Van Tooren, Surf. Sci. 48, 463 (1975), I. Forbeaux, J.-M. Themlin, and J.-M. Debever PHYSICAL REVIEW B VOLUME 58, NUMBER 24 (1998)] on graphitization of a silicon carbide surface, consists in obtaining a thin carbon layer on a SiC surface in vacuum conditions as a result of silicon sublimation at high temperatures reaching above 11000 C. At such temperatures, silicon evaporates from the surface, which, in turn, becomes rich in carbon. The carbon present on the surface is stable even in the form of one or two atoms layers. That is how graphene with thickness from several to tens of carbon atoms layers can be obtained. The growth rate of graphene is controlled by the production of the initial partial pressure of silicon in the reaction chamber, generated during SiC sublimation, and by subsequent conducting the process under conditions close to equilibrium. A variant of the method proposed in [K. V. Emtsev et al., Nat. Mater. 8: 203 (2009), W. Strupiński, et al, Mater. Science Forum Vols. 615-617 (2009)] enables graphene growth under the argon atmosphere at either reduced or atmospheric pressure. By adjusting the pressure (from 100 mbar to 1 bar) and the temperature of the process (from 1100° C. to 1800° C.), one controls the graphene growth rate. The described method is currently the most widely used one. Disadvantages of this method include: difficulty in obtaining the equilibrium pressure of Si in vacuum conditions, which limits its industrial use, and dependence of graphene quality on the quality of a SiC substrate out of which silicon sublimation occurs, which leads to inhomogenities in graphene parameters.
Yet another method is to deposit carbon atoms layers on metallic surfaces such as nickel, tungsten or copper. A commonly known CVD (Chemical Vapour Deposition) technique for deposition of thin films is applied in this case. Carbon sources include methane, propane, acetylene and benzene, all of which are decomposed at a high temperature. Released carbon deposits on a metallic substrate. In electronic applications, a subsequent indispensable step is to detach graphene from a conductive metal (by dissolving the metal in chemical reagents) and place it on an isolated substrate [Kim, K. S., et al., Nature 2009, 457, Reina, A., et al., J. Nano Lett. 2009, 9]. The method of graphene relocation has serious limitations that impede the industrial implementation. During relocation, graphene splits into smaller parts, Apart from that, the metal surface is not sufficiently smooth, when compared to the silicon carbide surface.
There are also two other methods of obtaining graphene consisting of chemical reduction of graphene oxide [Park, S.; Ruoff, R. S. Nat. Nanotechnol. 2009, 4, 217-224, Paredes, J. I.; Villor-Rodii, S., et al., Langmuir 2009, 25 (10), 5957-5968] as well as dissolution of graphite in solvents [Blake, P. Brimicombe, P. D. Nair, et al., Nano Left. 2008, 8 (6), 1704-1708, Hernandez, Y. Nicolosi, V. Lotya, et al. J. N. Nat. Nanotechnol. 2008, 3, 563-568] followed by evaporation of solid phase extraction of carbon in the form of thin flakes. However, graphene obtained by these methods is of particularly low quality.
In the case of epitaxy of carbon (CVD), SiC (silicon carbide) substrate, which, depending on the needs, is characterised by either high resistance (semi-insulating) or low resistance, proves attractive and suitable for electronic applications. (Conductive metallic substrates with a graphene layer make the manufacture of e.g. a transistor impossible). A CVD process requires high temperatures. The lower range is limited by the temperature of thermal decomposition of a gaseous carbon precursor (around 1000° C.); however, the growth of graphene of required structural quality needs to be performed at the temperatures in the range from 1500 to 1800° C. That temperature leads to SiC substrate decomposition or, in other words, silicon sublimation, which is disadvantageous from the point of view of epitaxy. Graphene growth by Si sublimation occurs (from about 1300° C.) before the temperature of epitaxial growth is achieved. Therefore, first carbon atoms layers, which are the most important for graphene parameters, will be formed by commonly known silicon evaporation, not by CVD epitaxy. Also, after CVD epitaxy is finished, uncontrolled sublimation will take place, causing further undesirable growth of successive carbon layers. Hence, the aim of the present invention is to propose a method for manufacturing graphene by vapour phase epitaxy (CVD), in which SiC substrates may be used owing to the control over the process of silicon sublimation from such a substrate. The present invention also aims to promote graphene nuclei growth on a SiC substrate by controlled silicon sublimation from this substrate and then deposition of epitaxial graphene layers on the thus obtained nuclei with a defined geometry (of an island). Application of CVD epitaxy for graphene manufacturing allows for the growth of thicker turbostatic graphene layers on a C-face (000-1) of a SiC substrate but also on a Si-face, which is not achievable in the case of Si sublimation. In addition, interrupting the CVD epitaxial growth and incorporating chemically reactive doponts enables the modification of graphene electron structure (energetic separation of Fermi level and the Dirac point). It is crucial however that the interruption of graphene growth is not followed by further uncontrolled process of sublimation or etching.